Use common.processutils.execute

This patch replaces the cinder.utils.execute method
with a wrapper around openstack.common.processutils.execute.

With the wrapper I'm also converting the processutils exceptions
to maintain compatability with the existing exceptions that are
caught/raised in the code.

Change-Id: Ibc36a1e83191e7c888ea62c55d28b7023535d9fa
Merge "Use common.processutils.execute"